Before pulp is formed into paper, it is almost invariably subjected to mechanical treatment in order to shorten, abrade, and internally fibrillate or bruise the structure of the fibers in the desired proportion and degree. The present method of accomplishing this is to make a slurry of the pulp with a concentration (consistency) of 3% to 5% and crush and rub the mixture between disks fitted with narrow metal bars disposed nearly radially. The relative speed of the bars is usually from 3,000 to 6,000 feet per minute and the pressure between the faces of the bars may be as high as 1,000 p.s.i. Consequently, the operation consumes a very large amount of energy for the resulting disruption of the fibrous structure. It is well recognized that, considering the results, the present method mechanically is a most inefficient operation.
An obvious approach to save energy which has no doubt been tried is to crush a layer of wet fibers between smooth rolls. However, it is not possible to feed any practical amount of wet fibers through such a nip, because water is squeezed from the pulp before it reaches the nip and the expressed water prevents the roll surface from grasping the pulp. Roughening the surfaces is no solution because the roughness would not much improve adhesion because the expressed water would tend to push and float the fibers away. Furthermore, the roughness would cut the fibers unduly, even if satisfactory feeding were possible.
It should be mentioned that with common barred machines, fiber clots are formed on the rapidly moving edges of the bars, which are further dewatered on the edges of the opposing bars when they come into contact. For effective treatment it is necessary that the clots of pulp be dewatered to about 50% dry before mechanical treatment.
Fifty years ago, I designed a laboratory apparatus constructed to beat a small batch of about three ounces of pulp prior to making it into test sheets of paper so as to evaluate its properties. The apparatus comprised three heavy rollers with 9-mm wide rims having 3 mm wide central grooves, the rollers restrained by gearing to rotate within a frame around a smooth, flat circular track around the bottom of a V-shaped annular trough. It and other beating devices have been described in detail by me in my textbook Pulp Technology and Treatment for Paper, pages 331 to 333, published by Miller Freeman in San Francisco in 1978 said disclosure being incorporated herein by reference.
Although the described laboratory apparatus was successful and has since been adopted as one of four instruments to carry out one of the official test methods of the Technical Association of the Pulp and Paper Industry (T225 OS-75), its principle of operation was not until recently fully understood; nor was it envisaged how the tiny laboratory batch apparatus could be modified and transformed into a simple, large, commercial machine having a continuous and substantial output of several tons an hour as is now described in the present invention.